1. Field of the Invention
This invention relates generally to power supply regulation and, more specifically, to inhibiting excessive current draw in a cable television distribution network.
2. Description of the Related Art
Cable television (cable TV or CATV) systems distribute signals from a head end to subscribers (xe2x80x9cdownstreamxe2x80x9d) through coaxial cable (xe2x80x9ccoaxxe2x80x9d), optical fiber, or a combination of coax and fiber commonly referred to as a hybrid network. Such cable network media are strung along telephone poles or through underground conduits.
Amplifiers and other devices are commonly included at intervals, which may be referred to as nodes, along the cable network. Amplifiers and similar devices commonly include active rather than passive electronics and thus require an external source of electrical power. In the case of a coax-based network, power is commonly supplied to each node through the same coax through which the radio frequency (RF) signals, e.g., television signals and digital signals, are carried. A device known as a power inserter or tap mixes power and RF signals for distribution on the coax. The voltage is typically 60 or 90 volts in U.S. cable networks. The power inserter commonly receives the power from a ferro-resonant converter, which in turn receives power from a transformer coupled to a power utility line.
At each network node at which an amplifier or other active device is located, a splitter separates the power from the RF signals. The splitter provides the power to the amplifier power supply. Power is recombined with the RF signals at the node, and the combined signal is placed on the coax for further transmission to downstream nodes. In the case of a hybrid network, the RF signals are carried on the fiber, and the power is carried on the coax; no power inserters or splitters are necessary.
Undesirable brownout or temporary low-voltage conditions may occur in a cable network. In a brownout condition, because the amplifier power supplies are constant-power devices, the dropping of the voltage carried on the coax drops causes the amplifier power supplies to draw more current. The additional current draw on the ferro-resonant converters can cause them to cease operating normally, i.e., cease resonating. Even after the voltage rises again to the normal level, a ferro-resonant converter may fail to re-start, i.e., begin resonating.
To avoid the above-described problem of the ferro-resonant converters failing to restart after a brownout, it has been suggested to include a low-voltage cut-off circuit at each node. A low-voltage cut-off circuit can disconnect an amplifier power supply from the coax when it detects a drop in the voltage below a threshold deemed to define such a brownout condition, such as 41 volts. The cut-off circuit may include hysteresis such that it does not reconnect the power supply until the voltage rises above a different threshold, such as 44 volts.
A problem with the use of low-voltage cut-off circuits is that they can cause undesirable oscillation or instability. The voltage at a node close to the head end may not drop sufficiently to cause a cut-off circuit at that node to disconnect a power supply from the coax supplying the power, but it may drop sufficiently at nodes farther downstream from the head end. As soon as the cut-off circuits at such nodes farther downstream from the head end detect a brownout and disconnect the power supplies from the coax, the voltage rises discontinuously at those disconnecting nodes as a result. The voltage rises discontinuously because no current exists in the coax downstream from the upstream-most disconnecting node. An absence of current implies a zero voltage drop across the resistance of the coax. Thus, the voltage at the disconnecting nodes instantaneously changes to the voltage at the upstream-most node that did not disconnect. Because this voltage is insufficient to cause the cut-off circuit at that node to disconnect, it may be similarly sufficient (i.e., but for hysteresis) to cause the cut-off circuits at nodes farther downstream to reconnect. But because the voltage at the reconnected nodes is still below the cut-off threshold, those nodes again disconnect. This disconnecting and reconnecting of cut-off circuits continues to occur in an oscillatory manner.
It would be desirable to provide a power supply circuit that does not cause oscillation in the event of a brownout condition and that does not hamper the operation of ferro-resonant converters. The present invention addresses these problems in the manner described below.
The present invention relates to power supply regulation and inhibiting excessive current draw by decreasing a power supply output voltage in response to a measured change in a parameter of the power supply input signal. The parameter can be either power supply input voltage or, alternatively, power supply input current. The power supply can be, for example, a cable television distribution network power supply that receives power from the coaxial cable that carries the television or other media signals or, in the case an optical fiber network, an auxiliary power cable associated with the fiber cable that carries the media signals.